Aromatic dicarboxylic acids are produced by the oxidation of dialkyl aromatics. Terephthalic acid (TPA) is commercially produced by the liquid phase oxidation of p-xylene in the presence of an oxidation catalyst such as a Co—Mn—Br and a solvent such as acetic acid. Isophthalic acid is produced by the oxidation of m-xylene. Both processes produce crude dicarboxylic acids containing colored impurities and mono-carboxylic acids such as carboxybenzaldehyde, i.e., 4-carboxybenzaldehyde (4-CBA) for terephthalic acid and 3-carboxybenzaldehyde (3-CBA) for isophthalic acid, and toluic acid, p-toluic acid for terephthalic acid and m-toluic acid for isophthalic acid. To achieve, for example, purified terephthalic acid used in the production of polyester fibers, films, and resins, crude terephthalic acid is treated further to remove impurities present due to the partial or incomplete oxidation of p-xylene. Typical commercial processes remove impurities by isolating a crude terephthalic acid solid, dissolving the solid in water at elevated temperatures and pressures, hydrogenating the resultant solution, cooling and crystallizing the product out of solution, and separating the solid product from the liquid. Colored impurities (of the benzil, anthraquinone, and fluorenone families) are hydrogenated to colorless products which are either are present in the TPA product or are removed in the the wastewater streams. Monofunctional 4-carboxybenzaldehyde is hydrogenated to p-toluic acid, which is separated from the sold product in the crystallization section of the process.
U.S. Pat. No. 4,158,738 describes a process for the production of terephthalic acid by the two-stage oxidation of p-xylene at elevated temperatures. This process requires a primary oxidation at temperatures greater than 210° C. Significant amounts of acetic acid, typically used as a solvent in the oxidation process, is oxidized at this temperature together with the p-xylene. The oxidation produces methyl acetate and methane, which must be disposed of, and carbon dioxide. The secondary oxidation of the process described in U.S. Pat. No. 4,158,738 occurs at a temperature equal to or less than that of the primary oxidation. The primary oxidation produces a crystalline, crude TPA product containing colored impurities and 4-CBA contained within the TPA crystals where it is difficult for the impurities to be further oxidized to TPA. The secondary oxidation temperature proposed by U.S. Pat. No. 4,158,738 does not allow for adequate dissolution of crude TPA crystals, and thus does not allow for sufficient conversion of the partially oxidized intermediate impurities, e.g., 4-CBA and p-toluic acid, contained therein to TPA.
U.S. Pat. No. 4,772,748 discloses a process for producing TPA by means of four oxidation steps, one of which is conducted at a higher temperature than the primary oxidation step. This higher temperature oxidation step uses molecular oxygen. Each of the oxidation steps is carried out using excess oxygen by controlling the flow of the air or other oxygen-containing gas to the oxidizer. However, feeding excess oxygen results in excessive oxidation (burn) of acetic acid. Furthermore, because the process uses four oxidation steps to reach the targeted impurity level, capital equipment and operating costs are high.